The present invention relates to an electron microscope and particularly to a method of controlling an electron optical condition of an irradiation lens system in a transmission electron microscope to reduce damage of a sample by an irradiation electron beam and a display apparatus therefor.
The transmission electron microscope is an apparatus by which an inner structure of a sample can be observed by focusing electrons passing though the sample or a lattice image can be observed using a diffraction wave. In recent years, the transmission electron microscope is used for structural analysis of biological materials such as protein, DNA, RNA and the like.
Up to now, in order to obtain a brightness of an enlarged image of a sample suitable for observation or picture-taking when the enlarged image is observed using a transmission electron microscope of a general two-stage irradiation lens type, the appropriate brightness has been set by changing a lens current of a second irradiation lens which is placed near the sample. Otherwise, adjustment of the brightness of the enlarged image has been performed by changing a bias voltage of a high voltage control unit to increase or decrease an electron beam current.
In the former method, when the brightness of the enlarged image is changed, an irradiated region (area) on the sample surface irradiated with the electron beam irradiating is changed and a portion (region) on the sample other than the region necessary for the observation or the picture-taking is irradiated with unnecessary electrons. Accordingly, the region of the sample surface unnecessary for the observation or the picture-taking may be also damaged. The electron beam irradiation damages the portion on the sample surface other than the region which has once been observed or formed into a picture, which may cause the next observation or picture-taking cannot be performed. That is, the efficiency of microscopic examination is extremely deteriorated by narrowing the region to be observed or to be formed into a picture, or by frequent exchange of the sample in some cases.
In the latter method, although the electron irradiated region on the sample surface is varied not so much, there has been a limitation in increasing the brightness of the enlarged image because the beam current cannot output above several tens xcexcA due to capacity of the high voltage circuit. Further, when the bias voltage is excessively lowered, an unsaturated image of the filament appears to occur unevenness in the brightness of the enlarged image though the beam current can be increased. Therefore, the changing of the brightness by changing the beam current is not practical due to the narrow variable range.
An object of the present invention is to provide a transmission electron microscope which can solve the above-mentioned problem, and can irradiate only a region of a sample necessary for observation or taking picture of the image with the electron beam with a necessary beam current to reduce the sample damage by unnecessary electron irradiation.
The above object of the present invention can be attained by providing an electron microscope with a function that relationship among an exciting current of each lens of an irradiation lens system including at least two stages of irradiation lenses and an electron beam aperture, an irradiation electron beam density onto a sample and an area of the sample surface irradiated with an electron beam is stored in a form of a table or an equation, and an exciting condition of each of the lenses of the irradiation lens system is retrieved from the relationship and set the irradiation lens system to the retrieved condition, for example, when the enlarging magnification is changed under a condition of keeping the irradiation electron beam density at a constant value or when the enlarging magnification is changed under a condition of keeping the brightness of a sample image at a constant value. Further, the above object of the present invention can be attained by that a region on a sample surface to be observed, for example, a region completely without sample damage with the electron beam can be efficiently selected by displaying trails of a region of the sample surface irradiated by the electron beam on a display unit.
That is, the present invention is characterized by an electron microscope comprising an electron beam source; at least two stages of irradiation lenses; an aperture capable of shading part of an electron beam emitted from the electron beam source; and a lens control means for controlling the irradiation lenses, a sample being irradiated with the electron beam focused by the irradiation lens and limited by the aperture, wherein the irradiation lens control means comprises a function to change a density of the irradiation electron beam while an area of an electron beam irradiation region on a sample surface is being kept to a nearly constant value.
Further, the present invention is characterized by an electron microscope comprising an electron beam source; at least two stages of irradiation lenses; an aperture capable of shading part of an electron beam emitted from the electron beam source; and a lens control means for controlling the irradiation lenses, a sample being irradiated with the electron beam focused by the irradiation lens and limited by the aperture, wherein the irradiation lens control means comprises a function to switch an area of an electron beam irradiation region on a sample surface in interlocking with an enlarging magnification of the electron microscope every time when the magnification is changed; and a function to change a density of the irradiation electron beam while the area of the electron beam irradiation region on the sample surface is being kept to a nearly constant value.
It is preferable that the irradiation lens control means comprises a function to convert an image acquisition region of an enlarged image, that is, a region of an enlarged image to be observed, recorded and displayed (typically, a region of an image capable of being acquired by a TV camera attached to the microscope) into a region on the sample surface using an enlarging magnification and to control so that the converted region becomes an electron beam irradiation region.
Further, the present invention is characterized by an electron microscope comprising an electron beam source; at least two stages of irradiation lenses; an aperture capable of shading part of an electron beam emitted from the electron beam source; and a lens control means for controlling the irradiation lenses, a sample being irradiated with the electron beam focused by the irradiation lens and limited by the aperture, wherein the irradiation lens control means comprises a function to change an area of an electron beam irradiation region while a density of the electron beam irradiating on a sample surface is being kept at a nearly constant value.
It is preferable that the irradiation lens control means comprises a function to switch said value of irradiation electron beam density kept at a nearly constant value in interlocking with an enlarging magnification of said electron microscope every time when the magnification is changed.
An electron microscope in accordance with the present invention comprises an input means for setting the irradiation electron beam density or for setting the area of the electron beam irradiating region.
Further, the present invention is characterized by an electron microscope comprising irradiation lenses and a sample fine movement apparatus, a sample surface being irradiated with an electron beam focused by said irradiation lenses, which further comprises a function for displaying a region of a sample surface irradiated with the electron beam based on information on the region irradiated by the electron beam and sample position information from the sample fine movement apparatus.
Furthermore, the present invention is characterized by an electron microscope comprising irradiation lenses and a sample fine movement apparatus, a sample surface being irradiated with an electron beam focused by said irradiation lenses, which further comprises a function for displaying a region of a sample surface irradiated by the electron beam with varying a display condition, for example, brightness (halftones) or display color corresponding to a quantity of irradiated electron beam based on a region irradiated with the electron beam, an irradiation electron beam density, an irradiation time period and sample position information from the sample fine movement apparatus.
An electron microscope in accordance with the present invention comprises a means for taking a picture of an enlarged image of the electron microscope and a means for displaying the picture of the image, and the display of the region of the sample surface irradiated with the electron beam may be displayed by overlapping onto the picture of the enlarged sample image.